The development of mine roof supporting systems has progressed from the use of wooden braces to roof bolting systems with elongate tensioned steel bolts and, more recently, to the grouting of bore holes.
As a seam of coal or ore is removed in the course of mining, the static vertical and horizontal pressures previously exerted by that seam no longer are available to maintain equilibrium within the surrounding strata. As a consequence, there occurs a slight heaving upward of the floor, an inward bulging at the sides and a drooping of the roof of the shaft. This vertical and horizontal displacement of the roof, sidewalls and floor occurs rapidly until a semi-static equilibrium is reached. However, a slow creep often will continue until the floor or a sidewall collapses.
Traditionally, roof bolting has been carried out by drilling a series of holes into a mine wall or roof strata, following which a steel anchor bolt with an expandable anchor on the upper end is inserted into each bore. A bearing plate is mounted on the lower end of the bolt to abut against the roof or wall surface. Next, the bolt is tightened both to lock the upper anchor and to tension the bolt, thus compressing the strata.
Recent theories describing roof collapse assert that a mere compression of the roof strata may not be adequate. The inadequacy stems from the side-slipping within the strata which may totally negate the support supplied by tensioned bolts. Such side-slipping can shift the direction of overburden pressures to negate the supportive effect of tensioned roof bolts. Thus, investigators have proposed that side-slipping of the overburden can be avoided by completely filling each roof bore with a dowel. As a consequence, as the stratum begins to shift, the incipient motion immediately will be opposed by the dowel surface. By comparison, conventional steel roof bolts are much slimmer than the roof bores in which they are inserted and strata slippage is permitted beyond a critical point leading to failure.
In the average underground coal mine, the seam of coal being excavated is about thirty inches in height. Thus, machines and apparatus used within such mines must be designed to operate under ceilings of that height. Accordingly, the current practice for inserting roof bolts is to flex the steel bolt through an angle of about 90.degree. before it is inserted within a roof bore. Generally, a thirty inch length of the bolt initially is inserted; then the bolt is straightened to the extent possible; the straightened part is inserted next; and this sequence continues until the bolt is completely inserted and the bearing plate and bolt head are in operable position.